1. Field of the Invention
The present invention relates to a gas sensor for detecting a component of a gas to be measured (hereinafter referred to as a measurement gas), such as an oxygen sensor, an HC sensor, an NO.sub.x sensor, or a like sensor.
2. Description of Related Art
Conventionally, there has been known a gas sensor is having a structure in which an insulator is disposed inside a metallic shell; a sensor element is disposed inside the insulator; and a sealing layer is formed between the inner surface of the insulator and the outer surface of the sensor element from a sealing material such as glass. The sealing layer is formed through charging of the sealing material into a cavity that is formed in the insulator around the sensor element.
In gas sensors having the above-described structure, the sensor element is frequently formed into a ceramic layered element. In such a case, as shown in FIG. 14, there is generally used a sensor element 102 having a rectangular cross section. However, a cavity portion 131 (i.e., a sealing material layer 132) formed in an insulator 104 conventionally has a circular cross section. This results in a structure in which the sealing material layer 132 having a circular cross section encloses the sensor element 102 having a rectangular cross section. Therefore, the thickness of the sealing material layer naturally becomes uneven such that the thickness is greater at portions corresponding to the longer sides of the cross section of the sensor element 102, and smaller at portions corresponding to the shorter sides of the cross section of the sensor element 102. For example, an oxygen sensor for automobile use is often mounted at a location, such as an exhaust manifold or an exhaust pipe located near a suspension system and tires, where the sensor may be exposed to high temperature or subjected to a strong thermal shock. When the sealing material layer 132 has an uneven thickness as described above, thermal stresses tend to concentrate at thin wall portions, resulting in the problem of shortening the life of the sealing material layer 132.
An object of the present invention is to provide a gas sensor in which the effect of thermal stress on a sealing-material layer is mitigated and which therefore has excellent durability.